Mass spectrometry is used for quantitative elemental analysis, identification of chemical structures and the determination of molecular weight and/or composition of mixtures. Mass spectrometry can be used to ascertain the molecular weights of molecules or the identity of components of a sample, based on the detection of a fragmentation pattern of ions produced when the material is ionized.
Mass spectrometry involves the formation of ions from analyte molecules, the separation of the various ions according to their mass-to-charge ratio (m/z) and the subsequent generation of a mass spectrum obtained from the separated ions as a result of their having passed through an electric field, a magnetic field or a combination thereof. In practice, positively and negatively charged ions are formed from a sample of molecules using, for example, electron impact, chemical ionization, atmospheric pressure ionization, fast atom bombardment, thermospray or electrospray techniques. The ions are accelerated to form an ion beam. Discrete mass fractions contained within the ion beam are then selected by a mass analyzer, such as a single-focusing or double-focusing mass analyzer, a time-of-flight mass analyzer or a quadrupole mass filter.
A mass spectrum of the ions can thus be produced and detected, providing a molecular fingerprint of the analyte molecule. The spectrum conveys information regarding the molecular weight of the molecule and, if fragmentation occurs during ionization, information characteristic of the position and bonding order of molecular substructures of the analyte molecule. In this way, a mass spectrum allows for the identification of molecules or compounds present within a sample.
An ion trap detector is a three-dimensional analog of a quadrupole mass filter, in which ion formation, storage and scanning operations may be performed within a single chamber. Typically, mass scanning is controlled by a radio frequency (rf) signal applied to a centrally located circular ring electrode disposed between two end-cap electrodes which are held at ground potential. Ions that have been formed inside the ion trap, or introduced therein from an associated ionizing means, are stored within the ion trap. Scanning operations can be conducted using a number of known methods to mass analyze stored ions. Ion traps can be used for conducting complex chemical and biochemical analyses of compounds, wherein high sensitivity, high mass range for molecular weight determination, and the capability for both mixture analysis and structural evaluation are important.
The conducting surfaces of the ring electrode and the two end-cap electrodes in an ion trap are ideally hyperbolic in cross section. The hyperbolic conducting surface of the ring electrode provides a first set of hyperbolic surfaces (Hyperbola set 1), and the hyperbolic conducting surfaces of the end-cap electrodes provide a second set of hyperbolic surfaces (Hyperbola set 2). The cross-sections of the two sets of hyperbolic surfaces should be complementary, and follow the general equations EQU Hyperbola set (1): r.sup.2 /(r.sub.0).sup.2 -z.sup.2 /(z.sub.0).sup.2 =1,
and EQU Hyperbola set (2): r.sup.2 /(r.sub.0).sup.2 -z.sup.2 /(z.sub.0).sup.2 =(-)1.
The machining settings for "r" and "z" that are used to fabricate the hyperbolic surfaces can be obtained using the Laplace condition EQU (r.sub.0).sup.2 =2(z.sub.0).sup.2,
with EQU r.sup.2 -2z.sup.2 -(r.sub.0).sup.2 =0;
and EQU r.sup.2 -2z.sup.2 +(r.sub.0).sup.2 =0.
The hyperbolic ring electrodes and end-cap electrodes of contemporary ion traps are typically fabricated from solid metals, such as stainless steel, using machining settings derived as explained above. Stepped surfaces result that are then finished using fine polishing. The selection of particular (r.sub.0) values depends upon the amplitude and frequency of the rf power supply to be employed and the desired mass range for detection. Once the metal electrodes have been machined, they must be insulated and spatially registered with each other using posts, washers and bands made from an insulating material such as ceramic to form an ion trap assembly. The high precision required of the complex machined hyperbolic surfaces result in high manufacturing costs. Further, the need to provide numerous external supports to assemble the ion traps, such as expensive ceramic insulating and/or registering components, increases material cost and results in labor intensive assembly of contemporary metal ion traps.
Accordingly, there remains a need in the art to provide an alternative ion trap electrode set which can be readily manufactured with high precision hyperbolic surfaces and assembled into a self-registering ion trap assembly.